Summary Conclusions and Outlook

Steiner et al. (2007) carried out a comparison of MSU/AMSU TLS temperatures from UAH and RSS, as well as of HadAT2 and ECMWF synthetic TLS temperatures, with synthetic TLS temperatures derived from CHAMP RO. Synthetic TLS temperatures were calculated by applying global weighting functions to RO temperature climatology profiles and alternatively by applying the RTTOV_8.5 radiative transfer model to the individual CHAMP RO profile data set. The analysis was performed for September 2001-December 2006 for four regions, almost global (70°S to 70°N), tropics (20°S to 20°N), NH extratropics (30°N to 70°N) and SH extrat-ropics (30°S to 70°S). Statistically significant trend differences between UAH and RSS to CHAMP TLS anomalies were found globally (-0.30 K/5 years to -0.36 K/5 years), stemming mainly from the tropics (-0.40 K/5 years to -0.42 K/5 years), whilst in the extratropics these cooling trend differences were not significant.

In this respect known error sources regarding the retrieval of RO data, the building of climatologies, and the related computation procedure of synthetic MSU temperatures were investigated and this study presented a closer analysis of these errors. The analysis yielded estimates of RO trend uncertainty over 5 years of < 0.02 K for potential initialization bias drifts, of < 0.02 K for dry/physical temperature difference, of about -0.1 K for TLS sampling error drift (significantly smaller in the tropics), and of < 0.02 K for weighting function/radiative transfer model uncertainty. The total contribution of these known error sources to the 5-year trend differences is about an order of magnitude smaller than the detected trend differences, and thus insufficient to account for them, especially in the tropical region.

The trend discrepancy is currently investigated in a follow on study and its resolution requires either additional, so far overlooked, sources of error in the RO TLS record or the presence of currently unresolved biases in the MSU records. Future inter-comparison of trends in RO data records of independent processing centers, and trace-back to the international time standard is planned to help quantify potential residual bias drifts.

For example, 2002-2006 RO records of four data centers (GFZ Potsdam, JPL Pasadena, UCAR/CDAAC Boulder, WegCenter Graz) indicated a temperature trend uncertainty amongst the four data sets of < 0.1 K/5 years at 10 km-30 km height in the regions studied here (Ho et al. analysis, B. Ho, UCAR Boulder, pers. communications, 2007). This is again much smaller than the trend differences found relative to MSU.

Furthermore, we found that SAC-C, GRACE, and COSMIC TLS temperatures closely match CHAMP temperatures, indicating the consistency and homogeneity of the RO data series.

The results underpin the benefit of having multiple independent estimates of the same variable from different instruments for detecting residual weaknesses in otherwise high-quality climate records. Continued inter-comparison, and exploiting the traceability of the RO data to the universal time standard (UTC), then enables us to further reduce the uncertainty in the climate records in absolute terms.

Acknowledgements The authors acknowledge GFZ (Potsdam, Germany) for the provision of CHAMP and GRACE RO data and ECMWF (Reading, UK) for access to their global operational analyses data. UCAR/CDAAC (CO, USA) is thanked for the provision of SAC-C and COSMIC data. UAH (AL, USA), RSS (CA, USA), and Hadley Centre/MetOffice (Exeter, UK) are acknowledged for the provision of MSU records. We thank J. Christy (UAH, AL, USA) for the provision of static weighting functions and his complementary helpful comments. We are grateful to H. Coleman (MetOffice/Hadley Centre, Exeter, UK) for her help regarding the HadAT2 data. The authors thank B. Ho (UCAR, Boulder, USA) for valuable discussions on the topic. The work was supported by the Austrian Science Fund (FWF; START Programme Y103-N03, Project INDICATE, P18733-N10, and Project CLIMROCC, P18837-N10). UF received financial support from the Max Kade Foundation (New York, NY, USA) and from UCAR.

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